Radio transmitter



Dec. 30, 1947. J, MI DODDS |51-Y AL 2,433,442

" RADIO TRANSMITTER Filed Deo. 29, 1945 AMPLIFIER alle: uylu- PatentedDec. 30, 1947 anni T OFFICE RADIO TRANSMTTER John Mathieson Dodds,Timperley, and .lohn

v Heywood Ludlow, Bowdon, England Application December 29, 1943, SerialNo. 516,106 In Great Britain May 13, 1938 (Cl. Z50-17) 3 Claims.

This invention relates to radio transmission systems.

Systems of radio communication have inherent limitations as to the speedand/or quality of the transmission of intelligence by reason of theirinability beyond a limit to produce and radiate high frequency signal-s,which, when received, can be resolved into satisfactory reproduction ofthe desired original signals. One limitation arises by reason of theuneven frequency response of the tuned circuits in the transmitter.Means such as overcoupling may be employed whereby improvements in theabove respects can be achieved when the wavelength is a few hundredmetres. The defect however is difficult to eliminate or even reduce onthe shorter wavelengths of say below 50 metres, especially intransmitters which are ar ranged for operation on several differentwavelengths.

The present invention provides a method by which the defect abovereferred to may be considerably reduced in a transmitter so that thespeed or quality or both of the transmission may be improved to a degreewhere it is substantially limited only by the receiving means which areemployed for the transmitted signals. Since the performance of thereceiver can be improved relatively cheaply, the present inventionprovides, inter alia, a saving in the cost of the system.

The present invention is concerned not so much with improving theperformance of transmitters modulated with relatively complex signalssuch as sound or vision modulation proper, but relates moreparticularly, although not exclusively to systems using instantaneouschanges of carrier level, such as the marking and spacing of code andthe impulses used in television for synchronisation.

The problem with which the present invention deals is as follows. Whenan attempt is made to excite a resonant circuit in conformation with asignal element of rectangular wave form such as is required in thesystems mentioned in the preceding paragraph, the resulting response inthe circuit is modified in a manner dependent on the resistance andcapacity in the circuit. It is convenient to consider the case where acircuit having resistance R and capacity C is required to respond to arapid succession of dots, that is square topped envelopes recurring atrelatively high frequency. As is well known, the level of the rst partof the dot will cause the radio frequency current to build upexponentially in the circuit asymptoting to the mark level whilst theend of the signal will initiate an exponential fall to the space levelwhich is usually zero. The time taken for, the current to rise or fallto a reasonable percentage of the nal value is dependent on the timeconstant RC of the circuit.

If this time is comparable With the duration of either mark or spacerequired in say high speed signalling, considerable distortion of thewave form results.

In short wave transmission the value of C is frequently irreduciblebelow the internal capacity of the power valves employed. Similarlythese valves for efcient operation require a certain minimum value of Rdetermined by the electrical characteristics of the valves. Thus thetime constant RC is more or less xedin value on those transmitters whichare the best suited for high speed operation.

The case under consideration is similar to that involved in picturemodulation in television transmission, Indeed the latter, which isnormally considered as a complex wave form containing all frequenciesfrom zero (direct current) up to the limit .frequency of the system (afew megacycles) might in the ideal embodiment cornprise va succession ofhorizontal steps occurring at various levels in accordance with thebrightness of successive picture elements joined by verticals, thelength of the steps corresponding with the size of the picture elementas is predetermined in the system. In practice this condition is notfulfilled nor with the existing limitations is it necessary, for sinceby analysis the stepped wave must cover an infinite frequency spectrum,the method would not be practicable. At present the upper limit offrequencies transmitted is between two and three megacycles. In orderto' achieve even this performance however, it is found necessary toreduce the effective value of R in the tuned circuits of the transmitterto a value far below the optimum from a point of view of the valvecharacteristics, with consequent poor eiciency. Thus it is found that ina transmitter requiring a frequency response at within three decibelsover a band of 1.5 megacycles the con- Version efficiency is less than15%: 60 kw. input being necessary to obtain 7.5 kw. output at 50megacycles carrier frequency: see Conklin and Gihring, Televisiontransmitters, R. C. A. Re- View, July 1937.

The rectangular wave forms now under consideration constitute, asmentioned above, a parallel or similar case with the foregoing and theobject of the presentI invention is to provide a satisfactory outputwave form with relatively great efficiency.

g ing in the aerial the tailing portion 3 (Fig. 2) as hereinbeforedescribed. This 'anti-phase burst must be so controlled that its energyis substantially wholly and solely utilised in augmenting the rate ofdecrease of the in-ph'a'se signal. It "is of course undesirable that anyof such anti-phase modulation should actually appear in the aerial.

It will be appreciated that the important parts of the additionalimpulses inserted by the device -29 (Fig. 3) are rstly the steepness ofthe impulse fronts and secondly the effective amplitudes thereof. In theexample given theimpulses are narrow rectangular ones, but apart fromthe two characteristics mentioned above the impulses need not conform tothis shape. In fact any impulses having steeper wave fronts and higherinitial amplitude than that produced by the signal alone will clearlyeiect an improvement. The particular circuit combinations used toprovide the distortion may thus be of simpler form than that requiredfor generating the narrow wave shape shown.

In many cases the correction afforded by the first impulse may beunnecessary whilst on the other hand the spacing of successive signalsWill render the second or anti-phase impulse desirable.

It will be appreciated that the advantage of the system above set forthand described is the improved degree of accuracy of transmissionobtainable. Moreover, with normal systems of keeping, wherein the spacedcondition is obtained by shutting off a valve by applying a largenegative potential to its grid, for example, the residual interelectrodecapacities of the valve or valves may still permit a small amount ofenergy to pass through them continuously. A further advantage,therefore, of the system lies in the fact that during the spacecondition both of the modulators 25 and 26 will pass such energy andtheir inputs being anti-phase but their outputs in parallel, no energyof lcarrier frequency will pass to the final amplier.

The use of the rst impulse, however, presupposes that the transmitter iscapable, at least for a Very short time, of producing considerably morepower than is required for the normal mark level. In some cases this maynot be considered economical, although it may prove desirable to employampliers which will give this condition in all the stages except theoutput stage. Some improvement will result from this arrangement.

It will be appreciated that the distorting device 29 must be arrangedfor the generation of impulses. In general, it may comprise a pair ofimpulse generators of which the output of one is connected between theleads 21 and 24a, whilst the output of the other is connected betweenthe leads,28 and 24a. These generators may for example be of the typeutilising switches such as grid-controlled discharge tubes and providedwith suitable resistance-capacity circuits to generate when switched ortriggeredf impulses of the required form. These two generators are soarranged that the rst is trigger-ed by the rising front of the signalelement which is applied to the terminals 33, after which the signalelement, with the impulse added to it is passed to the leads 2l and 24a,whilst the wave-front produced by the end of the signal element may beused to trigger the other impulse generator which thus produces asimilar impulse on the leads 28 and 2da. Fig. 4 illustratesdiagrammatically an arrangement for producing the impulses.

f* Referring te rig. il which is to be considered in eenunetian k'withFig.. f3, the tendu ers 2c and rildo'f the latter diagram have con-nated :across fthe resistance Rl which is of low value and Ttheconductor-'s '21 and 24d have #connected across 'er 'the switch bladesare adapted to be connected to a source of voltage V through acombination of rsistances and cndensers as follows. The switch terminal:i: is vadapted to be connected to the source V through the resistanceR3 which is bridged by a condenser C2, whilst the terminal y of theswitch S is adapted to be connected to the source V through theresistance R4 which is high compared with Rl, the latter being bridgedby the condenser Cl. It is assumed that the switch S is operatedelectromagnetically by means of a coil s connected to the terminals 33(Fig. 3) to which an electro-mechanical keying arrangement is connected.The arrangement shown in Fig. 4 operates as follows.

When no voltage is aplied to the terminals 33,4

the coil s' is de-energised and the switch S occupies the position shownso that in the steady condition the resistances R4 and Rl act as apotential divider for the lead 28 across the source V. Therefore, sincethe value Rl is low compared with that of R4, the voltage on the lead 28is near zero whilst the condenser C2 having been discharged by theresistance R3, the potential of the switch terminal .'11 is that of thesource V. When the voltage is applied to terminals 33, the switch bladesmove over to the other position so that the pot-ential of :c appears onthe conductor 21. However, owing to the condenser resistance combinationC2R3 the voltage on the conductor 21 drops exponentially to a steadylevel which will be RVi-R2 During this time the condenser CI is nolonger bridged by the low resistance Rl and so it charges up to thepotential V through the resistance R4. When the Voltage is removed fromthe terminals 33 the switch S rapidly moves over so that the potentialof the conductor 21 falls immediately to zero by reason of theresistance R2, but the potential V on the condenser Cl momentarily`appears on conductor 28, this potential falling exponentially to zeroas the condenser CI discharges through the resistance Rl.

We claim:

l. A radio transmission system arranged to transmit rapid changes incarrier level, means to produce an in-phase modulated carrier, means toproduce an anti-phase modulated carrier, means controlling the in-phaseand anti-phase carriers and arranged upon a rapid decrease in inputsignal to supply said anti-phase carrier producing means with an initialimpulse whereby the anti-phase carrier solely augments the rate ofdecrease of the in-phase carrier level.

2. A radio transmission system arranged to transmit rapid changes incarrier level, an aerial, means to produce an in-phase modulatedcarrier, means to produce an anti-phase modulator carrier, meanscontrolling the in-phase and antiphase carriers and responsive to a.rapid increase in input signal to superimpose thereon an initial impulseto augment the rate of increase in carrier -level in the aerial, saidlast-named means being effective upon a rapid decrease in input signalto supply said anti-phase carrier producing means with an initialimpulse to solely auglevel.

3. A radio transmission system as claimed in claim 2, wherein the meanscontrolling the inphase and anti-phase carriers is adjusted to cause theanti-phase carrier producing means to bring down the carrier level inthe aerial rapidly to substantially zero.

JOHN MATHIESON DODDS. JOHN HEYWOOD LUDLOW.

REFERENCES CITED The following references are of record in the ment therate of decrease of the in-phase carrier 5 me 0f thls patfent:

UNITED STATES PATENTS Number

